Our long-term objective is to reach an understanding of the auditory system that will allow quantitative modeling of the preception of tone, noise, and speech in both normal and impaired hearing. Such models not only will improve our understanding of normal auditory processes, but also will aid significantly in providing better diagnostic tools for hearing impairment and better rehabilitation of hearing-impaired listeners. The proposed project aims to obtain a coherent set of parametric data on successive and simultaneous envelope comparisons. Successive envelope comparisons are tested in experiments on discrimination of a difference in envelope (amplitude-modulation) frequency. Simultaneous envelope comparisons are tested in experiments on the effect of a co-modulated cue on envelope-frequency discrimination (i.e., Co-modulation Cued Envelope-Frequency Discrimination, CCEFD) and on masking (i.e., Co-modulation Masking Release, CMR). Based on forced-choice experiments in normal listeners, impaired listeners, and listeners with impairments simulated by masking a normal ear, we aim to establish a firm psychoacoustic basis for a model of normal and impaired listeners' processing of envelope fluctuations and envelope correlations in different auditory channels. Our specific aims are (1) To determine the roles spectral and temporal cues in explaining the qualitatively different results obtained in experiments on rate discrimination of filtered pulse trains and in experiments on envelope-frequency discrimination of two-tone complexes. Envelope-frequency discrimination will be compared for two-tone complexes, band-pass filtered pulse trains, and low- pass filtered pulse trains multiplied by a sinusoid. The low-pass filtered, multiplied pulse trains have envelopes similar to band- pass filtered pulse trains, but--like two-tone complexes--produce only small spectral changes for a given change in envelope frequency. (2) To test the hypotheses that listeners may be able to compare simultaneous modulation waveforms at different frequencies and use the information to improve discrimination of an envelope-frequency difference at one frequency in the presence of a co-modulated cue at another frequency. Several experiments on CCEFD with correlated cues, uncorrelated cues, and no cue will be performed. (3) To map the parameters that govern the efficiency of a correlated cue in CCEFD. Experiments are designed to explore systematically the effects of frequency separation between signal and cue, cue position, and cue level. (4) To test the hypothesis that the co-modulation advantage in CCEFD and CMR result from the same auditory process--direct comparison of envelopes at different frequencies. The effects of signal frequency and envelope frequency will be compared systematically in experiments on CCEFD and experiments on CMR. These experiments will aid in determining both the extent and the underlying causes for impaired listeners' probable deficits in envelope processing and provide important data for quantitative modeling of CCEFD and CMR in normal and impaired hearing.